Serum-mediated Haemonchus contortus larval aggregation differs by larval stage and is enhanced by complement.

Abstract

The objectives of this study were to measure Haemonchus contortus larval aggregation by complement/antibody complexes, determine effect of breed resistance and infection status and determine the effect of larval maturation on larval aggregation in vitro. Larval binding assays were performed on H.&#xa0;contortus L3, exsheathed L3 and L4 incubated with serum from either parasite na&#xef;ve or H.&#xa0;contortus primed St. Croix (resistant) and Suffolk (susceptible) lambs. No differences in L3 aggregation were observed between serum from either breed or infection status. Exsheathed L3 (60%) and L4 (42%) aggregation by primed Suffolk serum was significantly reduced compared with L3 (80%, P<.001). Removal of either complement or antibody effectively eliminated L3 aggregation (P<.001). Combination of antibody-depleted and complement-inactivated serum restored L3 aggregation to levels consistent with unprocessed serum, supporting a role for antibody and complement in aggregation (P<.001). Use of fluorescence-labelled anti-sheep IgG antibody allowed documentation of IgG bound to serum complexes within L3 masses and was present only in larvae incubated with normal serum, and complement- and antibody-depleted serum combination. These data indicate that complement/antibody complexes inhibit larval motility through larval aggregation which may be critical in early larval clearance of H.&#xa0;contortus.